The first three cases of the year are residents of Becker, Carver, and Clay counties.
The Minnesota Department of Health said the Clay County resident is a man in his sixties and the Becker and Carver county individuals are both under the age of 20.
They all experienced onset of symptoms in early to mid-July and were later diagnosed with West Nile fever, a milder form of WNV-related illness. All three people are now recovering.
Health department officials urged people to use mosquito repellent and to take other simple precautions against mosquito bites this season.
"Culex tarsalis, the primary mosquito carrier of West Nile virus in Minnesota, was at low levels earlier this summer when weather was cooler than normal, but its numbers have begun to rise with recent warm weather," said David Neitzel, an epidemiologist in a release from the department.
"Rising numbers of this mosquito species, the first human cases, and recent findings from mosquito and bird monitoring mean that the peak West Nile virus risk season has begun," he said.
Illness from West Nile virus can occur in residents throughout Minnesota and among all age groups. However, the risk is greatest in western and central counties, where Culex tarsalis mosquitoes are most common. Also, people who are elderly or who have weakened immune systems face the highest risk of developing severe illness from a WNV infection.
"We expect the risk of WNV-related illness to increase during late July and August as more Culex tarsalis mosquitoes are feeding," said Neitzel.
"This puts Minnesotans at risk for severe or even fatal illness," Neitzel added, "but people can reduce their chance of WNV infection by using mosquito repellents at dusk and dawn."
The health department said repellents containing DEET (up to 30 percent concentration) are still the most widely used and can provide long-lasting protection against mosquito bites.
For people who prefer not to use DEET, effective alternatives include repellents containing picaridin, oil of lemon eucalyptus, or IR3535.
Other important steps to prevent mosquito bites and serious illness from West Nile virus include:
-Minimizing outdoor activities at dusk and dawn, as this is prime feeding time for WNV-carrying mosquitoes. If you go outside at these times, take precautions even if mosquito numbers seem low; it only takes one bite from an infected mosquito to transmit the virus. -Wearing loose-fitting long-sleeve shirts and long pants if you must spend time in an area where mosquitoes are biting.
Since West Nile virus was first found in Minnesota in 2002, 441 cases (including 14 fatalities) of West Nile disease have been reported to the Minnesota Department of Health. Of those, 101 occurred in 2007.
Of those who become infected with WNV, most people will fight off the virus without any symptoms or will develop West Nile fever, the less severe form of the disease. Approximately 1 out of 150 people bitten by infected mosquitoes will develop severe central nervous system disease (encephalitis or meningitis).
Approximately 10 percent of people with this severe form of infection die from their illness, and many more suffer from long-term nervous system problems.
Symptoms usually show up 3 to 15 days after being bitten. They can include headache, high fever, rash, muscle weakness, stiff neck, disorientation, convulsions, paralysis and coma. Severe cases tend to occur most often in the elderly and people with immune system problems.